It is one of the most likely places in our solar system to harbour alien life, but the vast ocean hidden beneath the surface of Saturn's moon Enceladus has baffled scientists for years.

But physicists may now have unravelled how the icy world still has liquid water beneath its frozen crust – and it's caused by its fluffy core.

Researchers say rather than having a solid stone centre, like the Earth or our own moon, Enceladus may be formed around a core of boulders and ice.

The ocean hidden beneath the frozen surface of Enceladus (pictured) may owe its existence to the unusual core at the heart of Saturn's icy moon. A new model suggests Enceladus may have a 'fluffy core' of rubble

The gravitational pull of Saturn would cause this unconsolidated rubble to flex and move around, generating heat which would melt the ice above.

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This tidal heating, according to the scientists, would be enough to prevent the ocean below the surface from freezing solid.

SATURN IS DEVOURING ITS MOON

Stunning images sent back by the Cassini spacecraft has revealed how Saturn's icy moon, Enceladus, is slowly being eaten up by the gas giant's rings.

The 310 mile (500km) wide moon, which astronomers say may harbour alien life, has a network of geysers pumping out tiny chunks of ice-water at 800mph (1287km/h).

But these geysers won't be there forever as images show how long, sinuous, tendril-like structures near Enceladus are transferring material from the moon into Saturn's rings.

These ghostly tendrils have long been known to follow Enceladus in its orbit around the gas giant – but the images provided scientists the first opportunity to track their source.

The tendrils reach into Saturn's E ring - the ring in which Enceladus orbits - extending tens of thousands of miles away from the moon.

The theory, which was put forward by Dr James Roberts, a planetary scientist at Johns Hopkins University Baltimore, Maryland, may also mean the ocean on Enceladus is more hospitable than previously believed.

Scientists had previously believed Saturn's tiny moon needed extremely high concentrations of antifreeze agents such as ammonia to keep oceans from forming.

The existence of Enceladus's subsurface ocean has been suspected for some time since Nasa's Cassini spacecraft spotted plumes of vapour and ice shooting out from the moon's surface.

Measurements by Cassini have also detected several gigawatts of heat being emitted from beneath the surface.

Dr Roberts said: 'There are several reasons why a liquid water layer is suspected to exist beneath the ice shell.

'I find that fragmentation of the core increases tidal dissipation by a factor of 20, consistent with the long-term dynamically sustainable level, even when the interior is completely frozen, but only if the interior starts out warm and tidal heating is strong from the beginning.

'Although an ocean need not be present in order for the interior to experience significant tidal heating, all models that dissipate enough heat to prevent runaway cooling are also warm enough to have an ocean.

Scientists have long been baffled how the frozen world of Enceladus maintains a liquid ocean beneath the frozen crust. Plumes of ice and vapour suggest it exists, but with a solid core the heat produced would have been insufficient. Instead the new work suggests it may have a rubble and ice filled core (illustrated)

Enormous plumes of ice and vapour have been seen bursting out from the surface of Enceladus (pictured), which has suggested the distant moon is highly geologically active. The movement of loose rubble in the core as it orbits Saturn would produce enough heat to maintain a liquid ocean

'Tidal dissipation in the weak core provides an additional source of heat that may prevent a global subsurface ocean from freezing.'

Saturn has been found to have 62 moons orbiting around the giant gas planet. Many of these are frozen inactive worlds, while others show signs of tectonic activity.

Discovered in 1789 by astronomer William Herschel, Enceladus orbits around 147,500 miles from Saturn every 32.8 hours.

It is around 318 miles across, while temperatures on the surface rarely rise above -330°F (-201°C).

Yet despite this extreme cold, Enceladus appears to have an extremely young icy surface, which suggests it is geologically active.

Giant plumes of ice and water vapour have been seen shooting out into space from the surface.

This has suggested that beneath the scarred and icy surface, there is an ocean of liquid salty water which may even contain organic molecules.

Dr Roberts model suggests that a rubble filled core (like that shown in c and d), would be better able to produce enough heat to maintain an ocean than a solid core (like that shown in a and b)

However, with a solid rocky core, as was suspected, the moon would lose heat over millions of years and be unable to maintain this liquid layer.

According to Dr Roberts, whose work is published in the journal Icarus, a soft, rubble-filled core would allow sufficient movement to produce enough heat to keep this layer liquid.

He found that an ocean beneath the surface would help to regulate the temperature of the moon's interior, preventing runaway cooling or widespread melting.

He added that it is possible the moon undergoes freezing and thaw cycles which leads to a rocky ocean floor.

Dr Roberts said: 'If the ocean cyclically freezes and thaws, the resulting expansion of the core fragments may result in size-sorting in the outermost layer of the core.

'During freeze cycles, meter-sized and smaller fragments may be suspended in a layer of dirty ice a few km thick, just above the core.

'During thaw cycles, these fragments form a regolith on the seafloor.'